1. Field of Invention
The present invention relates generally to laser scanners of ultra-compact design capable of reading bar code symbols in point-of-sale (POS) and other demanding scanning environments.
2. Brief Description of the Prior Art
The use of bar code symbols for product and article identification is well known in the art. Presently, various types of bar code symbol scanners have been developed. In general, these bar code symbol readers can be classified into two distinct classes.
The first class of bar code symbol reader simultaneously illuminates all of the bars and spaces of a bar code symbol with light of a specific wavelength(s) in order to capture an image thereof for recognition and decoding purposes. Such scanners are commonly known as CCD scanners because they use CCD image detectors to detect images of the bar code symbols being read.
The second class of bar code symbol reader uses a focused light beam, typically a focused laser beam, to sequentially scan the bars and spaces of a bar code symbol to be read. This type of bar code symbol scanner is commonly called a xe2x80x9cflying spotxe2x80x9d scanner as the focused laser beam appears as xe2x80x9ca spot of light that fliesxe2x80x9d across the bar code symbol being read. In general, laser bar code symbol scanners are sub-classified further by the type of mechanism used to focus and scan the laser beam across bar code symbols.
The majority of laser scanners in use today, particular in retail environments, employ lenses and moving (i.e. rotating or oscillating) mirrors and/or other optical elements in order to focus and scan laser beams across bar code symbols during code symbol reading operations. In demanding retail scanning environments, it is common for such systems to have both bottom and side-scanning windows to enable highly aggressive scanner performance, whereby the cashier need only drag a bar coded product past these scanning windows for the bar code thereon to be automatically read with minimal assistance of the cashier or checkout personal. Such dual scanning window systems are typically referred to as xe2x80x9cbiopticalxe2x80x9d laser scanning systems as such systems employ two sets of optics disposed behind the bottom and side-scanning windows thereof. Examples of polygon-based bioptical laser scanning systems are disclosed in U.S. Pat. Nos. 4,229,588 and 4,652,732, assigned to NCR, Inc., each incorporated herein by reference in its entirety.
In general, prior art bioptical laser scanning systems are generally more aggressive that conventional single scanning window systems. For this reason, bioptical scanning systems are often deployed in demanding retail environments, such as supermarkets and high-volume department stores, where high check-out throughput is critical to achieving store profitability and customer satisfaction.
While prior art bioptical scanning systems represent a technological advance over most single scanning window system, prior art bioptical scanning systems in general suffered from various shortcomings and drawbacks.
In particular, the laser scanning patterns of such prior art bioptical laser scanning systems are not optimized in terms of scanning coverage and performance, and are generally expensive to manufacture by virtue of the large number of optical components presently required to constructed such laser scanning systems.
Thus, there is a great need in the art for an improved bioptical-type laser scanning bar code symbol reading system, while avoiding the shortcomings and drawbacks of prior art laser scanning systems and methodologies.
Moreover, the performance of such aggressive laser scanning systems (in scanning a bar code symbol and accurately produce digital scan data signals representative of a scanned bar code symbol) is susceptible to noise, including ambient noise, thermal noise and paper noise. More specifically, during operation of such machines, a focused light beam is produced from a light source such as a visible laser diode (VLD), and repeatedly scanned across the elements of the code symbol attached, printed or otherwise fixed to the object to be identified. In the case of bar code scanning applications, the elements of the code symbol consists of a series of bar and space elements of varying width. For discrimination purposes, the bars and spaces have different light reflectivity (e.g. the spaces are highly light-reflective while the bars are highly light-absorptive). As the laser beam is scanned across the bar code elements, the bar elements absorb a substantial portion of the laser beam power, whereas the space elements reflective a substantial portion thereof. As a result of this scanning process, the intensity of the laser beam is modulated to in accordance with the information structure encoded within the scanned bar code symbol. As the laser beam is scanned across the bar code symbol, a portion of the reflected light beam is collected by optics within the scanner. The collected light signal is subsequently focused upon a photodetector within the scanner which generates an analog electrical output signal which can be decomposed into a number of signal components, namely a digital scan data signal having first and second signal levels, corresponding to the bars and spaces within the scanned code symbol; ambient-light noise produced as a result of ambient light collected by the light collection optics of the system; thermal noise produced as a result of thermal activity within the signal detecting and processing circuitry; and xe2x80x9cpaperxe2x80x9d or substrate noise produced as a result of the microstructure of the substrate in relation to the cross-sectional dimensions of the focused laser scanning beam. The analog scan data signal has positive-going transitions and negative-going transitions which signify transitions between bars and spaces in the scanned bar code symbol. However, as a result of such noise components, the transitions from the first signal level to the second signal level and vice versa are not perfectly sharp, or instantaneous. Consequently, it is difficult to determine the exact instant that each binary signal level transition occurs in detected analog scan data signal.
It is well known that the ability of a scanner to accurately scan a bar code symbol and accurately produce digital scan data signals representative of a scanned bar code symbol in noisy environments depends on the depth of modulation of the laser scanning beam. The depth of modulation of the laser scanning beam, in turn, depends on several important factors, namely: the ratio of the laser beam cross-sectional dimensions at the scanning plane to the width of the minimal bar code element in the bar code symbol being scanned, and (ii) the signal to noise ratio (SNR) in the scan data signal processing stage where binary level (1-bit) analog to digital (A/D) signal conversion occurs.
As a practical matter, it is not possible in most instances to produce analog scan data signals with precisely-defined signal level transitions. Therefore, the analog scan data signal must be further processed to precisely determine the point at which the signal level transitions occur.
Hitherto, various circuits have been developed for carrying out such scan signal processing operations. Typically, signal processing circuits capable of performing such operations include filters for removing unwanted noise components, and signal thresholding devices for rejecting signal components which do not exceed a predetermined signal level.
One very popular approach for converting analog scan data signals into digital scan data signals is disclosed in U.S. Pat. No. 4,000,397, incorporated herein by reference in its entirety. In this U.S. Letters Patent, a method and apparatus are disclosed for precisely detecting the time of transitions between the binary levels of encoded analog scan data signals produced from various types of scanning devices. According to this prior art method, the first signal processing step involves double-differentiating the analog scan data input signal Sanalog to produce a second derivative signal Sxe2x80x3analog. Then the zero-crossings of the second derivative signal are detected, during selected gating periods, to signify the precise time at which each transition between binary signal levels occurs. As taught in this U.S. patent, the selected gating periods are determined using a first derivative signal Sxe2x80x2analog formed by differentiating the input scan data signal Sanalog. Whenever the first derivative signal Sxe2x80x2analog exceeds a threshold level using peak-detection, the gating period is present and the second derivative signal Sxe2x80x3analog is detected for zero-crossings. At each time instant when a second-derivative zero-crossing is detected, a binary signal level is produced at the output of the signal processor. The binary output signal level is a logical xe2x80x9c1xe2x80x9d when the detected signal level falls below the threshold at the gating interval, and a logical xe2x80x9c0xe2x80x9d when the detected signal level falls above the threshold at the gating interval. The output digital signal Sdigital produced by this signal processing technique corresponds to the digital scan data signal component contributing to the underlying structure of the analog scan data input signal Sanalog.
While the above-described signal processing technique generates a simple way of generating a digital scan data signal from a corresponding analog scan data signal, this method has a number of shortcomings and drawbacks.
In particular, thermal as well as xe2x80x9cpaperxe2x80x9d or substrate noise imparted to the analog scan data input signal Sanalog tends to generate zero-crossings in the second-derivative signal Sxe2x80x3analog in much the same manner as does binary signal level transitions encoded in the input analog scan data signal Sanalog. Consequently, the gating signal mechanism disclosed in U.S. Pat. No. 4,000,397 allows xe2x80x9cfalsexe2x80x9d second-derivative zero-crossing signals to be passed onto the second-derivative zero-crossing detector thereof, thereby producing erroneous binary signal levels at the output stage of this prior art signal processor. In turn, error-ridden digital data scan data signals are transmitted to the digital scan data signal processor of the bar code scanner for conversion into digital words representative of the length of the binary signal levels in the digital scan data signal. This can result in significant errors during bar code symbol decoding operations, causing objects to be incorrectly identified and/or erroneous data to be entered into a host system.
Also, when scanning bar code symbols within a large scanning field with multiple scanning planes that cover varying focal zones of the scanning field, as taught in co-applicant""s PCT International Patent Publication No. WO 97/22945 published on Jun. 26, 1997, Applicants"" have observed that the effects of paper/substrate noise are greatly amplified when scanning bar code symbols in the near focal zone(s), thereby causing a significant decrease in overall system performance. In the far out focal zones of the scanning system, Applicants have observed that laser beam spot speed is greatest and the analog scan data signals produced therefrom are time-compressed relative to analog scan data signals produced from bar code symbols scanned in focal zones closer to the scanning system. Thus, in such prior art laser scanning systems, Applicants"" have provided, between the first and second differentiator stages of the scan data signal processor thereof, a low-pass filter (LHF) having cutoff frequency which passes (to the second differentiator stage) the spectral components of analog scan data signals produced when scanning bar code elements at the focal zone furthest out from the scanning system. While this technique has allowed prior art scanning systems to scan bar codes in the far focal zones of the system, it has in no way addressed or provided a solution to the problem of increased paper/substrate noise encountered when scanning bar code symbols in the near focal zones of such laser scanning systems.
Moreover, although filters and signal thresholding devices are useful for rejecting noise components in the analog scan signal, such devices also limit the scan resolution of the system, potentially rendering the system incapable of reading low contrast and high resolution bar code symbols on surfaces placed in the scanning field.
Thus, there is a great need in the art for improved laser scanning system wherein the analog scan data signals generated therewithin are processed so that the effects of thermal and paper noise encountered within the system are significantly mitigated while not compromising the scan resolution of the system.
Accordingly, a primary object of the present invention is to provide a novel bioptical laser scanning system which is free of the shortcomings and drawbacks of prior art bioptical laser scanning systems and methodologies.
Another object of the present invention is to provide a bioptical laser scanning system, wherein a plurality of pairs of quasi-orthogonal laser scanning planes are projected within predetermined regions of space contained within a 3-D scanning volume defined between the bottom and side-scanning windows of the system.
Another object of the present invention is to provide such a bioptical laser scanning system, wherein the plurality of pairs of quasi-orthogonal laser scanning planes are produced using at least one rotating polygonal mirror having scanning facets that have high and low elevation angle characteristics.
Another object of the present invention is to provide such a bioptical laser scanning system, wherein the plurality of pairs of quasi-orthogonal laser scanning planes are produced using at least two rotating polygonal mirrors, wherein a first rotating polygonal mirror produces laser scanning planes that project from the bottom-scanning window, and wherein a second rotating polygonal mirror produces laser scanning planes that project from the side-scanning window.
Another object of the present invention is to provide such a bioptical laser scanning system, wherein each pair of quasi-orthogonal laser scanning planes comprises a plurality of substantially-vertical laser scanning planes for reading bar code symbols having bar code elements (i.e., ladder type bar code symbols) that are oriented substantially horizontal with respect to the bottom-scanning window, and a plurality of substantially-horizontal laser scanning planes for reading bar code symbols having bar code elements (i.e., picket-fence type bar code symbols) that are oriented substantially vertical with respect to the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system comprising a plurality of laser scanning stations, each of which produces a plurality of groups of quasi-orthogonal laser scanning planes that are projected within predetermined regions of space contained within a 3-D scanning volume defined between the bottom and side-scanning windows of the system.
Another object of the present invention is to provide a bioptical laser scanning system, wherein two visible laser diodes (VLDs) disposed on opposite sides of a rotating polygonal mirror are used to create a plurality of groups of quasi-orthogonal laser scanning planes that project through the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system, wherein a single VLD is used to create the scan pattern projected through the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system which generates a plurality of quasi-orthogonal laser scanning planes that project through the bottom-scanning window and side-scanning window to provide 360-degrees of scan coverage at a POS station.
Another object of the present invention is to provide a bioptical laser scanning system which generates a plurality of vertical laser scanning planes that project through the bottom-scanning window to provide 360 degrees of scan coverage.
Another object of the present invention is to provide a bioptical laser scanning system which generates a plurality of horizontal and vertical laser scanning planes that project from the top of the side-scanning window downward, which are useful for reading ladder type and picket-fence type bar code symbols on top-facing surfaces.
A further object of the present invention is to provide such a bioptical laser scanning system, in which an independent signal processing channel is provided for each laser diode and light collection/detection subsystem in order to improve the signal processing speed of the system.
A further object of the present invention is to provide such a bioptical laser scanning system, in which a plurality of signal processors are used for simultaneously processing the scan data signals produced from each of the photodetectors within the laser scanner.
A further object of the present invention is to provide a bioptical laser scanning system that provides improved scan coverage over the volume disposed between the two scanning windows of the system.
Another object of the present invention is to provide a bioptical laser scanning system that produces horizontal scanning planes capable of reading picket-fence type bar code symbols on back-facing surfaces whose normals are substantially offset from the normal of the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces horizontal scanning planes that project from exterior portions (for example, left side and right side) of the side-scanning window at a characteristic propagation direction whose non-vertical component is greater than thirty-five degrees from normal of the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces vertical scanning planes capable of reading ladder type bar code symbols on back-facing surfaces whose normals are substantially offset from the normal of the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces a plurality a vertical scanning planes that project from portions (e.g., back-left and back-right corners) of the bottom-scanning window proximate to the back of the bottom-scanning window and the bottom side of the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces vertical scanning planes capable of 360 degree reading of ladder type bar code symbols (e.g., on bottom-, front-, left-, back- and/or right-facing surfaces of an article).
Another object of the present invention is to provide a bioptical laser scanning system that produces a plurality a vertical scanning planes that project from each one of the four corners of the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least eight different vertical scanning planes that project from the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 13 different horizontal scanning planes that project from the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 20 different horizontal scanning planes that project from the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 21 different scanning planes that project from the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 28 different scanning planes that project from the side-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least seven different vertical scanning planes that project from the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 21 different horizontal scanning planes that project from the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system that produces at least 25 different scanning planes that project from the bottom-scanning window.
Another object of the present invention is to provide a bioptical laser scanning system with at least one laser beam production module that cooperates with a rotating polygonal mirror and a plurality of laser beam folding mirrors to produce a plurality of scanning planes that project through the window, wherein the incidence angle of the laser beam produced by the laser beam production module is offset with respect to the axis of rotation of the rotating polygonal mirror.
A further object of the present invention is to provide such a bioptical laser scanning system wherein the offset of the incidence angle of the laser and the axis of rotation of the rotating polygonal mirror produces overlapping scanning ray patterns that are incident on at least one common mirror to provide a dense scanning pattern projecting therefrom.
In another aspect of the present invention, it is a primary objective to provide an improved laser scanning system, wherein scan data signals produced therewithin are processed so that the effects of thermal and paper noise encountered within the system are significantly mitigated.
Another object of the present invention is to provide an improved laser scanning system having a scan data signal processor with improved dynamic range.
Another object of the present invention is to provide an improved laser scanning system having a multi-path scan data signal processor that employs different operational characteristics (such as different filter cutoff frequencies, peak thresholds, etc) in distinct signal processing paths.
Another object of the present invention is to provide an improved laser scanning system having a multi-path scan data signal processor that concurrently performs distinct signal processing operations that employ different operational characteristics (such as different filter cutoff frequencies, peak thresholds, etc).
Another object of the present invention is to provide an improved laser scanning system employing a scan data signal processor having a plurality of processing paths each processing the same data signal derived from the output of a photodetector to detect bar code symbols therein and generate data representing said bar code symbols, wherein the plurality of processing paths have different operational characteristics (such as different filter cutoff frequencies, peak thresholds, etc).
A further object of the present invention is to provide such an improved laser scanning system wherein each signal processing path includes a peak detector that identifies time periods during which a first derivative signal exceeds at least one threshold level, and wherein the at least one threshold level for one of the respective paths is different than the at least one threshold level for another of the respective paths.
A further object of the present invention is to provide such an improved laser scanning system wherein each signal processing path performs low pass filtering, wherein the cut-off frequency of such low pass filtering for one of the respective paths is different than the cut-off frequency of such low pass filtering for another of the respective paths.
A further object of the present invention is to provide such an improved laser scanning system wherein each signal processing path performs voltage amplification, wherein the gain of such voltage amplification for one of the respective paths is different than the gain of such voltage amplification for another of the respective paths.
Another object of the present invention is to provide an improved laser scanning system employing a scan data signal processor with dynamic peak threshold levels.
Another object of the present invention is to provide an improved laser scanning system employing a scan data signal processor with multiple signal processing paths that perform analog signal processing functions with analog circuitry.
Another object of the present invention is to provide an improved laser scanning system employing a scan data signal processor with multiple signal processing paths that perform digital signal processing functions with digital signal processing circuitry.
A further object of the present invention is to provide such a laser scanning system, wherein each processing path is performed sequentially based on real-time status of a working buffer that stores data values for digital signal processing.
These and other objects of the present invention will become apparent hereinafter and in the Claims to Invention.